In a wide variety of application, there exists the need for products which will thicken or gel organic solvent based formulations. There also is a need for products to be used in the areas of organic-water system applications as gelling materials therefor.
Previous developments of Teng et al (one of the present inventors) have shown that esters of hydroxypropyl cellulose and starch are useful as gelling agents for organic solvents.
In Teng et al U.S. Pat. No. 3,730,693 are disclosed lipophilic polymeric carbohydrate derivatives as gelling agents, specifically cellulose laurate and starch laurate.
In Teng et al U.S. Pat. No. 3,870,701 are disclosed benzyl hydroxypropyl cellulose acetate gelling agents and Teng et al Ser. No. 387,894 filed Aug. 13, 1973, now U.S. Pat. No. 3,940,384, discloses methyl hydroxypropyl cellulose acetates as gelling agents.
Teng et al U.S. Pat. No. 3,824,085 discloses methods for producing acetate esters of hydroxypropyl cellulose which are effective gelling agents for a series of commercially important solvents. One of the reactants required for production of these esters is acetic anhydride. Because of the low yield of the acetylation reaction, it is desirable to find a reactant that has a higher percentage conversion than acetic anhdyride.
We have developed another series of effective gelling agents; namely, methyl hydroxpropyl cellulose ethers (MHPC) having a degree of substitution (D.S.) of methyl groups of 1.0 to 2.4 and a degree of molar substitution (M.S.) of hydroxypropyl groups of 2 to 8. We have found that methyl hydroxypropyl cellulose ethers gel a larger variety of organic solvents than do the acetate esters of hydroxypropyl cellulose.
The purpose of the following paragraph is to explain the use herein of the terms degree of substitution (D.S.) and degree of molar substitution (M.S.).
The degree of substitution is defined as the average number of hydroxyl groups substituted per anhydroglucose unit. The maximum number of hydroxyl groups per anhydroglucose unit is three, and therefore the theoretical maximum degree of substitution is also three in the case of monofunctional substituents. In the case of polyfunctional or polymerizable substitutents that can react not only with the hydroxyl groups of the polysaccharide but also with themselves, the number of substituents is no longer limited by the three available hydroxyl groups on the anhydroglucose unit. The term degree of molar substitution, (M.S.), is adopted and defined simply as the number of moles of substituent per anhydroglucose unit. There is no theoretical maximum value for the degree of molar substitution, (M.S.).
Methyl hydroxypropyl cellulose ethers have been synthesized by the hydroxypropylation of cellulose, followed by a methylation step. Although methyl hydroxypropyl cellulose ethers are disclosed in U.S. Pat. No. 2,831,852, the methyl hydroxypropyl cellulose ethers of the present invention are clearly distinguishable from those of U.S. Pat. No. 2,831,852 due to the different M.S. of hydroxypropyl groups and D.S. of methyl groups. Furthermore, methyl hydroxypropyl cellulose ethers of the present invention show solubility and gelling characteristics that are not possible with those of the methyl hydroxypropyl cellulose ethers of U.S. Pat. No. 2,831,852 when combined in organic solvents.
A comparison of certain of the chemical and physical properties of the methyl hydroxypropyl cellulose ethers of U.S. Pat. No. 2,831,852 and the methyl hydroxypropyl cellulose ethers of the present invention are shown in the following Table I.
TABLE I __________________________________________________________________________ Solubility M.S. of D.S. Of Carbon Hydroxypropyl Methyl Tetra- Group Group Water chloride Toluene __________________________________________________________________________ Methyl hydroxy- propyl cellulose ether of Pat. No. clear insol- insol- 2,831,852 0.3-1 1.5-2.0 gel uble uble Methyl hydroxy- propyl cellulose ethers of present cloudy clear clear invention 2-8 1.0-2.4 metastable gel gel gel __________________________________________________________________________
These gelling agents provide many desirable properties which are lacking in the presently available gelling agents. Generally, methyl hydroxypropyl cellulose ethers display greater clarity in gelling solutions than do hydroxypropyl cellulose acetates. Methyl hydroxypropyl cellulose ethers also gel with solvent-water mixtures, whereas hydroxypropyl cellulose acetates do not. Methyl hydroxypropyl cellulose ethers can gel the widest range of diverse solvents as listed in Table I. Furthermore, the ether linkages of methyl hydroxypropyl cellulose ethers are more stable to alkali, acid, and water than the ester linkages of hydroxypropyl cellulose acetates.
The preparation of these materials is economical, based on both material and processing costs. The reactions are run under mild conditions with no special equipment required except a pressure reactor. The reactants include cellulose, sodium hydroxide, propylene oxide, and methyl chloride (or methyl bromide or dimethyl sulfate). The use of methyl chloride is most desirable in that it is practical and efficient. Any of a number of inert solvents may be used, e.g., toluene, hexane, dimethyl formamide, dioxane. Hexane is preferred for the reason that product recovery is simplified.
The products of this invention are lipophilic polymers capable of thickening or gelling a wide variety of solvents.
Examples of solvents which are capable of being gelled with methyl hydroxypropyl cellulose ethers are seen in Table II. These organic solvents may be esters, ketones, aromatic hydrocarbons, nitriles, amides, alcohols or halogenated solvents with a solubility parameter of about 8 to about 16.
The solubility parameter is a measure of the compatibility of solutes with solvents and its definition and determinations are set forth in Polymer Handbook, edited by E. H. Immergut, Interscience Publishers (1966). The solubility parameter, ".delta.," is a thermodynamic property of solvents. Thermodynamic calculations show that when a solute is mixed with a solvent of equal solubility parameter, spontaneous dissolution takes place. Once the value for a given polymer is determined, it is known that other solvents with comparable values will also dissolve it.
The term solubility as used in this context has a somewhat different meaning than when it is used conventionally. Solubility is used generally to indicate the extent of interaction between a solid and a solvent. A piece of solid, when placed in a solvent, will dissolve into the solvent until the saturation point is reached. At that point, the two phases, solid and liquid, coexist at equilibrium. The amount of solute in liquid is measured as the solubility of the material in solution. However, in polymers and particularly in the case of the gelling agent of this invention, there is no obvious saturation point. When immersed in a `compatible` solvent, the gelling agents swell and dissolve. When the solvent concentration is high, a polymer solution forms; when the solvent concentration is low only swelling and hence gelling occurs. An apparent single phase (solution or gel) is reached at all times. To examine qualitatively the compatibility of a gelling agent with a solvent, 5 grams of gelling agent is placed in 100 ml. of solvent. If only one phase is observed (gel or solution) they are compatible. When the mixture retains two phases, they are incompatible.
Table II shows examples of solvents with their corresponding solubility parameters. The methyl hydroxypropyl cellulose ethers gel solvents with solubility parameters of 8 to 16.
TABLE II ______________________________________ Solvents Solubility Parameter ______________________________________ ethyl acetate 8.4 carbon tetrachloride 8.4 toluene 8.9 methyl ethyl ketone 9.3 dioxane 10.3 pyridine 10.3 acetonitrile 11.5 dimethyl formamide 12.1 methanol 14.5 methyl formamide 16.1 ______________________________________
We have found that methyl hydroxypropyl cellulose ethers are particularly useful in gelling or thickening organic solvents at concentrations from 0.4 to 5% (W/W). The gelling agents of this invention are soluble in a wide range of organic solvents and water solvent mixtures, and are effective thickeners or gellants at 1% concentrations.
Solutions and gels may be prepared by simple agitation and heating.